U.S. patent number 6,891,167 [Application Number 09/882,818] was granted by the patent office on 2005-05-10 for apparatus and method for applying feedback control to a magnetic lens.
This patent grant is currently assigned to KLA-Tencor Technologies. Invention is credited to John A. Notte, IV.
United States Patent |
6,891,167 |
Notte, IV |
May 10, 2005 |
**Please see images for:
( Certificate of Correction ) ** |
Apparatus and method for applying feedback control to a magnetic
lens
Abstract
An apparatus configured to control a magnetic field strength of
a magnetic lens is provided. The apparatus may include a magnetic
sensor configured to generate an output signal responsive to a
first magnetic field strength of the magnetic lens. The apparatus
may also include a control circuit coupled to the magnetic sensor
and the magnetic sensor. The control circuit may be configured to
receive the output signal from the magnetic lens and to receive an
input signal responsive to a predetermined magnetic field strength.
The control circuit may be further configured to generate a control
signal responsive to the output signal and the input signal.
Additionally, the control circuit may be configured to apply a
current to the magnetic lens such that a second magnetic field
strength may be generated within the magnetic lens closer to the
predetermined magnetic field strength than the first magnetic
strength.
Inventors: |
Notte, IV; John A. (Windham,
NH) |
Assignee: |
KLA-Tencor Technologies
(Milpitas, CA)
|
Family
ID: |
22789580 |
Appl.
No.: |
09/882,818 |
Filed: |
June 15, 2001 |
Current U.S.
Class: |
250/396ML;
250/396R; 250/397 |
Current CPC
Class: |
H01J
37/34 (20130101); H01J 37/141 (20130101); B82Y
15/00 (20130101); H01J 2237/2817 (20130101); H01J
2237/2485 (20130101); H01J 2237/3175 (20130101); H01J
2237/30405 (20130101) |
Current International
Class: |
H01J
37/28 (20060101); H01J 37/252 (20060101); H01J
1/02 (20060101); H01J 1/08 (20060101); H01J
037/252 (); H01J 037/28 () |
Field of
Search: |
;250/396ML,396R,397,398,310,309 ;315/8 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report, application No. PCT/US01/19437, mailed
Oct. 11, 2001..
|
Primary Examiner: Wells; Nikita
Assistant Examiner: Quash; Anthony
Attorney, Agent or Firm: Mewherter; Ann Marie Daffer
McDaniel, L.L.P.
Parent Case Text
This application claims benefit of U.S. Provisional 60/212,104
filed Jun. 15, 2000.
Claims
What is claimed is:
1. An apparatus configured to control a magnetic field strength of
a magnetic lens during use, comprising: a magnetic sensor disposed
within a magnetic field generated by the magnetic lens, wherein the
magnetic sensor is further disposed within the magnetic lens,
wherein the magnetic sensor is configured to generate an output
signal during use, wherein the output signal is responsive to a
first magnetic field strength of the magnetic field generated by
the magnetic lens, and wherein the magnetic field will be applied
to a charged particle beam traveling through the magnetic lens; and
a control circuit coupled to the magnetic sensor and the magnetic
lens, where in the control circuit is configured: to receive the
output signal from tho magnetic sensor during use; to receive an
input signal responsive to a predetermined magnetic field strength
during use; to generate a control signal responsive to the output
signal and the input signal during use; and to apply a current to
the magnetic lens, wherein the current is responsive to the control
signal.
2. The apparatus of claim 1, wherein the magnetic lens is coupled
to a scanning electron microscope.
3. The apparatus of claim 1, wherein the input signal comprises a
voltage having a linear relationship to the predetermined magnetic
field strength of the magnetic lens.
4. The apparatus of claim 1, wherein the output signal comprises a
voltage having a linear relationship to the first magnetic field
strength of the magnetic lens.
5. The apparatus of claim 1, wherein the control signal is further
responsive to a function of the output signal and the input
signal.
6. The apparatus of claim 1, wherein the control circuit is further
configured to apply a current to at least one coil of the magnetic
lens.
7. The apparatus of claim 1, wherein the current is effective to
generate a second magnetic field strength within the magnetic lens,
and wherein the second magnetic field strength is closer to he
predetermined magnetic field strength than the first magnetic field
strength.
8. The apparatus of claim 1, wherein the current is effective to
generate a second magnetic field strength within the magnetic lens,
and wherein the second magnetic field strength is substantially the
same as the predetermined magnetic field strength.
9. The apparatus of claim 1, wherein the apparatus is further
configured to continuously control the magnetic field strength of
the magnetic lens during use.
10. The apparatus of claim 1, wherein the apparatus is further
configured to intermittently control the magnetic field strength of
the magnetic lens during use.
11. The apparatus of claim 1, wherein the magnetic sensor is
further disposed within a cavity in the magnetic lens, and wherein
the cavity is disposed between an outer pole piece of the magnetic
lens and an inner pole piece of the magnetic lens.
12. The apparatus of claim 1, wherein the magnetic sensor is
further disposed within an inner pole piece of the magnetic
lens.
13. The apparatus of claim 1, further comprising a temperature
sensor coupled to the magnetic lens, wherein the temperature sensor
is configured to generate a temperature signal during use, and
wherein the temperature signal is responsive to a temperature of
the magnetic lens.
14. The apparatus of claim 13, wherein the temperature sensor is
further coupled to the magnetic sensor, wherein the magnetic sensor
is further configured to receive the temperature signal during use,
and wherein the output signal is further responsive to the
temperature of the magnetic lens.
15. The apparatus of claim 1, wherein the control circuit comprises
a low-pass circuit element configured to receive the output signal
during use and to reduce fluctuations in the output signal during
use.
16. The apparatus of claim 1, wherein the control circuit comprises
an operational amplifier configured to generate a comparison signal
during use, wherein the comparison signal is responsive to a
comparison of the output signal and the input signal, and wherein
the control signal is further responsive to a function of the
comparison signal.
17. The apparatus of claim 1, wherein the control circuit comprises
an electronic current drive system configured to receive the
control signal during use and to apply the current to the magnetic
lens during use.
18. A method for controlling a magnetic field strength of a
magnetic lens, comprising: generating an output signal in response
to a first magnetic field strength of a magnetic field generated by
the magnetic lens, wherein the magnetic field will be applied to a
charged particle beam traveling through the magnetic lens, and
wherein said generating is performed by a magnetic sensor disposed
within the magnetic lens; generating an input signal in response to
a predetermined magnetic field strength; generating an input signal
in response to the output signal and the input signal; and applying
a current to the magnetic lens, wherein the current is responsive
to the control signal.
19. A system configured to inspect a specimen during use,
comprising: at least one magnetic lens configured to apply a
magnetic field to a charged particle beam during use, wherein the
magnetic lens is positioned along a path of the charged particle
beam; and an apparatus configured to control a magnetic field
strength of the magnetic field generated by the magnetic lens
during use, wherein the apparatus is coupled to the magnetic lens
and the system, the apparatus comprising: a magnetic sensor
disposed within the magnetic field generated by the magnetic lens,
wherein the magnetic field will be applied to the charged particle
beam traveling through the magnetic lens, wherein the magnetic
sensor is further disposed within the magnetic lens, wherein the
magnetic sensor is configured to generate an output signal during
use, and wherein the output signal is responsive to a first
magnetic field strength of the magnetic field generated by the
magnetic lens; and a control circuit coupled to the magnetic sensor
and the magnetic lens, wherein the control circuit is configured:
to receive the output signal from the magnetic sensor during use;
to receive an input signal responsive to a predetermined magnetic
field strength during use; to generate a control signal responsive
to the output signal and the input signal during use; and to apply
a current to the magnetic lens, wherein the current is responsive
to the control signal.
20. A method for inspecting a specimen, comprising: generating a
magnetic field by a magnetic lens and applying the magnetic field
to a charged particle beam, wherein applying the magnetic field to
the charged particle beam comprises directing the charged particle
beam through the magnetic lens; and controlling a magnetic field
strength of the magnetic field of the magnetic lens, comprising:
generating an output signal in response to a first magnetic field
strength of the magnetic field generated by the magnetic lens,
wherein said generating is performed by a magnetic sensor disposed
within the magnetic lens; generating an input signal in response to
a predetermined magnetic field strength; generating a control
signal in response to the output signal and the input signal; and
applying a current to the magnetic lens, wherein the current is
responsive to the control signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a magnetic lens which may be
configured to apply a magnetic field to a charged particle beam,
and more particularly, to a sectored magnetic lens and a control
apparatus for a magnetic lens which may be incorporated into a
scanning electron microscope system.
2. Description of the Related Art
As the dimensions of semiconductor devices continue to shrink with
advances in semiconductor materials and processes, the ability to
examine microscopic features and to detect microscopic defects has
become increasingly important in the successful fabrication of
advanced semiconductor devices. Significant research continues to
focus on increasing the resolution limit of metrology tools that
are used to examine microscopic features and defects. Optical
microscopes generally have an inherent resolution limit of
approximately 200 nm and have limited usefulness in current
manufacturing processes. Microscopes that utilize electron beams to
examine devices, however, may be used to investigate feature sizes
as small as, e.g., a few nanometers. Therefore, tools that utilize
electron beams to inspect semiconductor devices are increasingly
becoming integral to semiconductor fabrication processes. For
example, in recent years, scanning electron microscopy has become
increasingly popular for the inspection of semiconductor devices.
Scanning electron microscopy generally involves scanning an
electron beam over a specimen and creating an image of the specimen
by detecting electrons that are reflected, scattered, and/or
transmitted by the specimen.
The electron optical system of a scanning electron microscope
generally includes an electron source, a device or a plurality of
devices configured to focus an electron beam generated by an
electron source, a detector or a plurality of detectors configured
to detect electrons reflected, scattered, or transmitted by the
specimen, and a control system. A thermal field emission source may
typically be used as an electron source, and the energy of the
electron source may be controlled by an emission control electrode
and an anode. The electron beam may pass through a magnetic
condenser lens configured to collimate the electron beam. An
initial deflection system may also be located near the electron
source. An initial deflection system may be configured to correct
alignment, stigmation and blanking of the beam. Prior to passing
through a magnetic objective lens, the beam may also be passed
through a beam limiting aperture and one or more electrostatic
pre-lens deflectors. The magnetic objective lens may further focus
the electron beam to a spot size of, for example, approximately
five nanometers. As used herein, the term "spot size" is generally
defined as a lateral dimension of an electron beam incident upon a
specimen. A magnetic objective lens may typically include a lower
pole piece, an intermediate electrode, and an upper pole piece.
An electron beam exiting a magnetic objective lens may be scanned
across a specimen. Typically, the electron beam may be scanned in a
first direction while the stage supporting the specimen may be
moved in a direction perpendicular to the first direction. A
plurality of detection systems may be used to detect secondary
electrons, back-scattered electrons, and transmitted electrons that
may be produced when the electrons contact the specimen. Examples
of scanning electron microscope systems are illustrated, for
example, in U.S. Pat. No. 4,928,010 to Saito et al., U.S. Pat. No.
5,241,176 to Yonezawa, U.S. Pat. No.5,502,306 to Meisburger et al.,
U.S. Pat. No. 5,578,821 to Meisburger et al., U.S. Pat. No.
5,665,968 Meisburger et al., U.S. Pat. No. 5,717,204 to Meisburger
et al., U.S. Pat. No. 5,869,833 to Richardson et al., U.S. Pat. No.
5,872,358 to Todokora et al., and U.S. Pat. No. 5,973,323 to Adler
et al., and are incorporated by reference as if fully set forth
herein.
The performance of a scanning electron microscope may vary
depending on, for example, the capability to focus an electron beam
on a small target area. High voltage electrons may penetrate deep
into a semiconductor substrate or a portion of a semiconductor
formed upon a semiconductor substrate thereby damaging the
substrate or the device and rendering it unsuitable as a working
device such as an integrated circuit. Therefore, low voltage
electron beams may typically used to analyze delicate semiconductor
specimens that otherwise might be damaged by high voltage electron
sources. The primary factor that reduces resolution in the low
acceleration voltage region is blur of the electron beam due to
chromatic aberration. Dispersion in the energy of the electron beam
emitted from the electron source typically causes chromatic
aberration. As such, significant effort has been focused on
improving the performance of a scanning electron microscope by
enhancing the ability of the magnetic objective lens to reduce
chromatic aberrations in an electron beam source especially in low
voltage particle beams.
Traditionally, magnetic lenses may be axially symmetric and may
produce axially symmetric magnetic potentials and magnetic fields.
An example of such a magnetic lens is illustrated, for example, in
U.S. Pat. No. 6,002,135 to Veneklasen et al. and is incorporated by
reference as if fully set forth herein. A magnetic lens may include
an inner pole piece that may have a cylindrical upper portion and a
conical lower portion that may be substantially enclosed by an
outer pole piece. The outer pole piece may also have a cylindrical
upper portion and a conical lower portion corresponding to the
inner pole pieces. A solenoidal excitation coil may be disposed
between the inner pole piece and the outer pole piece. When a
current is applied to the excitation coil, an axial focusing field
may be generated within the lens by magnetic flux from the inner
and outer pole pieces. The axial focusing field may be used to
focus an electron beam. Shielding rings may be arranged between the
upper and lower portions of the inner pole piece to reduce the air
gap between the pole portions. The shielding rings may also provide
a return path for deflection flux that may otherwise radiate
through the gap and induce eddy currents in outer pole pieces and
excitation coil. Deflection coils may also be included within the
lens along the beam path.
Variable axis lenses have also been developed to focus electron
beams. Variable axis lenses incorporate supplementary lenses or
supplementary deflectors in the magnetic lens to provide some
correction of electron beam paths that may be laterally displaced
from an optical axis of the lens. The supplementary lenses and
deflectors may be energized based on the lateral displacement of
the beam path. Although electron beams may be deflected by this
lens, astigmation may still be a problem. Therefore, a separate
astigmation compensator may also be included in such a lens.
Alternatively, an astigmatism-correction deflector system may be
arranged within a variable axis lens adjacent the internal surface
of the supplementary deflectors. Such deflectors may be constructed
of an octapole three-stage coil in which each octapole includes two
tetrapole sets. A deflection field coil may be added to one of the
tetrapole coil sets of the octapole. An example of a variable axis
lens is illustrated in U.S. Pat. No. 5,952,667 to Shimizu and is
incorporated by reference as if fully set forth herein. The
incorporation of a separate astigmator octapole forces the beam to
pass through the center of this octapole. The overall alignment of
the lens system, however, may be non-colinear due to the
incorporation of such a separate feature. Therefore, complexity of
the overall alignment of the system increases when the charged
particle beam is forced to pass through successive non-colinear
points.
There are, however, several disadvantages to the lens systems
described above. For example, axially symmetric lenses may
typically suffer from hysteresis, large inductance of the
excitation coil, and thermal stability problems. Hysteresis may
cause a relationship between the excitation coil current and the
deflected beam position to depend upon past deflection history.
Therefore, accurate focus of an electron beam using the magnetic
lens may be extremely difficult to maintain and control.
Additionally, large inductance of the excitation coil may cause
thermal stability problems due to heat generated by the lens or the
excitation coils. Therefore, the center of the lens may shift due
to thermal expansion of the materials used to construct the
lens.
Immersion lenses are also limited in their application to a variety
of specimens. Immersion lenses are generally designed to limit
aberrations of an electron beam by reducing the distance between
the specimen and the maximum magnetic field. The distance between
the specimen and the maximum magnetic field may be reduced by
placing the specimen near or within the lens. Examples of immersion
lenses are illustrated in U.S. Pat. No. 5,089,428 to Da Lin et al.
and is incorporated by reference as if fully set forth herein. Due
to space limitations, immersion lenses may not be able to
accommodate a large specimen such as a semiconductor substrate. For
example, 200 mm wafers, or semiconductor substrates, are already
being used in the development and production of semiconductor
devices. Efforts are also underway to further increase the size of
semiconductor substrates to 300 mm. Modifying these lenses in order
to accommodate such large semiconductor substrates may also
adversely affect the performance of immersion lenses.
Alternatively, reducing the size of the semiconductor substrate by
cross-sectioning the wafer is not usually an option due to the cost
associated with destroying a product wafer.
An asymmetric immersion lens may be configured to reduce the
distance between a specimen and the strongest magnetic field of the
lens. An asymmetrical lens, however, may be configured to produce a
magnetic field that rises sharply just in front of a conical pole
piece near the bore of the lens or the position at which the
electron beam exits the magnetic lens. The magnetic field falls
slowly toward a second pole piece or a magnetic housing. The
specimen and the conical pole piece are disposed within the
magnetic housing such that the specimen may be placed near the
conical pole piece. Asymmetric immersion lenses may be more
flexible to accommodate large specimen such as semiconductor
substrates, but these lenses may have a reduced capability to
detect secondary electrons. For example, because secondary
electrons may be emitted at a point beyond the magnetic field peak,
low energy secondary electrons may not be able to surmount the
magnetic field maximum. In order to overcome low detection of
secondary electrons, a conducting grid system may be included in
the lens. The conducting grid system may include an auxiliary grid
to accelerate the secondary electrons away from the inner surface
of the lower pole piece, an extraction grid to reduce the axial
velocity of the secondary electrons, and a restrain grid to turn
back any uncollected secondary electrons. Therefore, in order to
overcome the low detection of secondary electrons, a conducting
grid system may be included in the lens.
In addition to the above disadvantages, the performance of magnetic
lenses may also be limited due to changes in the magnetic field
strength due to low frequency noise, drift in the performance of
current drive electronics, drift due to eddy currents or
superimposed fields from other sources, and drift in the magnetic
field strength over time from other causes. Although a magnetic
lens design may minimize these effects on the performance of the
lens, it may not be possible to substantially eliminate magnetic
field drift of the lens. For example, eddy currents due to magnetic
flux leakage from a lens through a gap in the magnetic lens may
adversely affect the performance of the magnetic lens. Because a
magnetic lens must be designed with a bore to allow the electron
beam to travel through the lens, however, it is impossible to seal
the magnetic lens off completely. As a result, some of the magnetic
field will inherently "leak" out of a magnetic lens. Therefore, the
effects of eddy currents on the performance of a magnetic lens may
not be completely eliminated due to usage requirements. Drift in
the magnetic field may cause the electron beam to drift out of
focus. Therefore, the overall resolution of a scanning electron
microscope may also be reduced by the presence of the above sources
of magnetic field drift. The functioning of the scanning electron
microscope may, however, be dramatically improved by an accurate
control system for the magnetic objective lens.
A control mechanism for a magnetic lens may generally include a
device for sensing the current density of the electron beam at a
position spaced from an axis along which an electron beam travels.
For example, an alignment yoke disposed along the axis of the
electron beam may receive a signal from the current sensing
arrangement. Therefore, the alignment yoke may be mechanically
shifted incrementally and orthogonally until a maximum current may
be produced at the reference location. The beam may be centered due
to the altered position of the alignment yoke thereby reducing
aberrations in the projections lens. An example of such a focusing
system is illustrated in U.S. Pat. No. 4,423,305 to Pfeifer and is
incorporated by reference as if fully set forth herein. Mechanical
focus methods, however, may be slow due to the time required for
moving the devices. In addition, microscopic vibrations due to
mechanical motion of the alignment yoke may need to settle before
the magnetic lens may perform adequately. Therefore, mechanical
focus methods may require additional time such that microscopic
vibrations will not affect the performance of the magnetic
lens.
An alternative control mechanism for a magnetic lens involves
focusing and controlling an electron beam by determining the focal
length at which a sample will be brought into focus. Focal length
may be a function of the electron beam energy and the magnetic
field strength. In this manner, one available control mechanism
involves using an electron trajectory tracing program to measure
the converging point, or focal length, for an electron beam by
using measurements of the electron beam energy and the magnetic
field strength. The magnetic field strength may be estimated by
measuring a current in the lens coil. An adjustment to the current
in the lens coil may be made to correct the converging point of the
electron beam. Instantaneous magnetic field strength, however, may
be determined by the present current and the history of all other
currents in the coil which is commonly referred to as hysteresis.
Hysteresis in the magnetic field strength may also be induced from
frequent changes in the voltage supplied to a magnetic lens. For
example, the lens current in a scanning electron microscope may
often be automatically adjusted to a nominal beam voltage. As new
specimens are being observed, the user may usually alter the beam
voltage to bring the specimen into focus. Therefore, frequently
adjusting the beam voltage may increase the complexity of a current
history of the lens. As the complexity of the current history
increases, hysteresis will also become more problematic. Therefore,
estimating the magnetic field strength using measurements of the
current in the lens coil may induce error in this method. In
addition, the coil and the core materials may react in a non-ideal
way to the frequent changes in the current being supplied to the
coil. Therefore, additional electrical and magnetic
characterization of the coil and the core materials may be
necessary. A thorough degaussing procedure may reduce the effects
of hysteresis, however, magnetic hysteresis typically remains a
problem in most magnetic lenses.
Additional methods to control the electron beam focus have
attempted to reduce the effects of hysteresis of the magnetic lens
by keeping the lens current constant after an initial manual focus
and calibration. An example of such a magnetic lens control method
is illustrated in U.S. Pat. No. 4,999,496 to Shaw et al. and is
incorporated by reference as if fully set forth herein. The control
method involves varying the electron beam energy to alter the focus
of the magnetic lens, as working distance changes such as when
different areas of a specimen are viewed. In order to offset the
effects of a new beam energy, the current in the scanning coils may
be altered to maintain accurate magnification. Although such a
method for focusing the electron beam may reduce deleterious
effects of hysteresis in the current of the magnetic objective
lens, other factors that may lead to defocusing may not be
addressed in this design. For example, as mentioned above, other
factors that may hinder the performance of a magnetic lens may
include thermal changes in the material properties, drift in the
current drive electronics, drift in the magnetic field due to eddy
currents, and drift due to superimposed fields from other
sources.
Furthermore, in many scanning electron microscope systems, coarse
and fine focusing of the magnetic lens may typically be performed
manually by an operator. The operator may alter the focus of the
magnetic lens by controlling the electric current of the magnetic
lens. In order to obtain the desired magnetic field strength, the
operator may alter the current while observing the effects of the
magnetic field on an image of a specimen until the optimal
performance is achieved. The image may be observed using a display
system such as a color or grayscale monitor. The resulting magnetic
field, however, may follow a nonlinear relationship with the
current due to hysteresis in the magnetic lens and the behavior of
the coil and the core materials in response to the change in
current. This iterative method also depends on operator judgement
and is consequently subject to error. More importantly, the
additional sources of magnetic field drift described above may also
cause this magnetic field to be irreproducible.
Accordingly, it would be advantageous to improve the performance of
magnetic lens and a magnetic lens control apparatus that may be
used to focus an electron beam by reducing the effects of, for
example, hysteresis and thermal instability in the magnetic field
strength of a magnetic lens.
SUMMARY OF THE INVENTION
In an embodiment, a magnetic lens such as a magnetic circuit
configured to apply a magnetic field to a charged particle beam and
an apparatus configured to control a magnetic field strength of a
magnetic lens are provided. The charged particle beam may be a beam
of ions or a beam of electrons. The charged particle beam may
travel through the magnetic lens from a first end of the magnetic
lens to a second end of the magnetic lens. The magnetic lens and
the apparatus may be configured to be incorporated into a scanning
electron microscope.
In an embodiment, the magnetic lens may have an outer pole piece
and an inner pole piece. The outer pole piece may be coupled to the
inner pole piece. The outer pole piece may have at least two
sectors and at least two slots. In addition, the outer pole piece
may have eight sectors and eight slots. Each sector may be disposed
between lateral boundaries of two slots in the outer pole piece
such that the magnetic potential of each sector may be
substantially independent of the magnetic potential of each other
sector on the outer pole piece. Furthermore, the inner pole piece
may also have at least two sectors and at least two slots. In
addition, the inner pole piece of the magnetic lens may have eight
sectors and eight slots.
In an embodiment, the magnetic lens may include a primary coil
winding. The primary coil winding may be interposed between the
outer pole piece and the inner pole piece of the magnetic lens. The
primary coil winding may be configured to drive a magnetic
potential of the outer pole piece relative to the inner pole piece
when a current is applied to the primary coil winding. The magnetic
lens may also include at least two sector coil windings. Each
sector coil winding may be coupled to one sector of the outer pole
piece. In addition, if the inner pole piece also has sectors, then
a sector coil winding may be coupled to each sector of the inner
pole piece. In this manner, the magnetic lens may have an equal
number of sectors and sector coil windings. Each sector coil
winding may be configured to drive a magnetic potential of the
sector coupled to each sector coil winding, respectively, when a
current is applied to each sector coil winding. Therefore, the
magnetic potential of each sector may include the magnetic
potential generated by the current applied to primary coil winding
which may affect each sector substantially equally. The magnetic
potential of each sector may also include the magnetic potential
generated by the current applied to each respective sector coil
winding. As such, each sector may have a magnetic potential that
may be substantially independent of the magnetic potentials of the
other sectors of the magnetic lens. The magnetic field that may be
applied to the charged particle beam, therefore, may include the
magnetic potential of the outer pole piece relative to the inner
pole piece and the magnetic potential of one or more sectors of the
magnetic lens.
In an embodiment, a method for applying a magnetic field to a
charged particle beam may include directing the charged particle
beam through a magnetic lens. Directing the charged particle beam
may include positioning a charged particle beam source configured
to generate a charged particle beam in substantial alignment with a
first end of the magnetic lens. The charged particle beam may
travel from a first end of the magnetic lens to a second end of the
magnetic lens. The magnetic lens may be configured as described in
an above embodiment. The method may also include applying a first
current to a primary coil winding of the magnetic lens to drive a
magnetic potential of an outer pole piece of the magnetic lens
relative to an inner pole piece of the magnetic lens. In addition,
the method may include applying a second current to at least one
sector coil winding of the magnetic lens to drive a magnetic
potential of at least one sector of the magnetic lens. Therefore,
the magnetic field that may be applied to the charged particle beam
may include the magnetic potential of the outer pole piece relative
to the inner pole piece and the magnetic potential of at least one
sector of the magnetic lens.
In an embodiment, a method for focusing a charged particle beam on
a specimen is provided. The specimen may be a semiconductor device
that may be fabricated using a semiconductor fabrication process.
Alternatively, the specimen may be a portion of a semiconductor
device or another specimen such as a biological sample. The method
may include positioning at least a portion of the specimen in a
path of the charged particle beam. The charged particle beam may be
directed through a magnetic lens from a first end of the magnetic
lens to a second end of the magnetic lens. The magnetic lens may be
configured as described in an above embodiment. Focusing a charged
particle beam on a specimen may also include applying a first
current to a primary coil winding of the magnetic lens to generate
a magnetic potential of an outer pole piece of the magnetic lens
relative to an inner pole piece of the magnetic lens. In addition,
focusing a charged particle beam on a specimen may include applying
a second current to one sector coil winding of the magnetic lens to
generate a magnetic potential of one or more sectors of the outer
pole piece of the magnetic lens. In this manner, the magnetic field
that may be applied to the charged particle beam may include the
magnetic potential of the outer pole piece relative to the inner
pole piece and the magnetic potential of one or more sectors of the
magnetic lens. The method may also include altering the magnetic
potential of the outer pole piece to apply a coarse focus
adjustment to the charged particle beam. Furthermore, the method
may include altering the magnetic potential of one or more sectors
to apply a fine focus adjustment to the charged particle beam.
In an embodiment, a system that may be used to inspect a
semiconductor device is provided. The semiconductor device may be
fabricated on a semiconductor substrate using a semiconductor
manufacturing process. The system may be a scanning electron
microscope that may use an electron beam to inspect the
semiconductor device. The system, however, may be any system that
may use a charged particle beam such as a beam of electrons or a
beam of ions to inspect a semiconductor device. The system may
include a charged particle beam source configured to generate a
charged particle beam. The system may also include at least one
magnetic lens that may be configured as described herein to apply a
magnetic field to the charged particle beam. The magnetic lens may
be positioned along a path of the charged particle beam generated
by the charged particle beam source. As such, the charged particle
beam may pass through the magnetic lens from a first end of the
magnetic lens to a second end of the magnetic lens. The system may
further include a stage configured to support at least a portion of
the semiconductor device. The stage may be positioned along the
path of the charged particle beam such that the charged particle
beam may interact with the semiconductor device. The charged
particle beam may interact with the semiconductor device subsequent
to having a magnetic field applied to the charged particle beam by
the magnetic lens.
In an additional embodiment, a method for inspecting a specimen
such as a semiconductor device is provided. Inspecting the specimen
may include positioning at least a portion of the specimen on a
stage. The stage may be configured to support the specimen and may
be positioned along a path of the charged particle beam. The method
may include generating a charged particle beam. The generated
charged particle beam may be directed through at least one pole
piece of a magnetic lens such that the charged particle beam may
travel from a first end of the magnetic lens to a second end of the
magnetic lens. The magnetic lens may be configured as described
herein and may be incorporated into a scanning electron microscope.
The method may further include applying a first current to a
primary coil winding to generate a magnetic potential of an outer
pole piece of the magnetic lens relative to an inner pole piece of
the magnetic lens. In addition, the method may include applying a
second current to a sector coil winding of the magnetic lens to
generate a magnetic potential of at least one sector of the outer
pole piece. Therefore, the magnetic field applied to the charged
particle beam may include the magnetic potential of the outer pole
piece and the magnetic potential of one or more sectors of the
outer pole piece.
An additional embodiment relates to a computer-implemented method
for controlling a magnetic field that may be applied to a charged
particle beam. The computer-implemented method may be implemented
by controller software that may be executable on a controller
computer. The controller computer may be coupled to a magnetic
lens. The method for controlling a magnetic field may also be
implemented by program instructions that may be incorporated into a
carrier medium. The method may include measuring a magnetic field
generated within a magnetic lens that may be configured as
described herein. The charged particle beam may be directed through
the magnetic lens such that the magnetic field of the magnetic lens
may be applied to the charged particle beam.
The method may also include determining a primary current in
response to the measured magnetic field. The primary current may be
applied to a primary coil winding that may be coupled to a pole
piece of the magnetic lens. In this manner, a magnetic potential of
the pole piece may be generated. In addition, the method may
include determining a secondary current in response to the measured
magnetic field. The secondary current may be applied to at least
one sector coil winding coupled to one sector of the pole piece. As
such, a magnetic potential of each sector of the pole piece may be
generated. Therefore, the magnetic field applied to the charged
particle beam may include the magnetic field strength of the pole
piece of the magnetic lens and the magnetic field strength of one
or more sectors of the magnetic lens. The method may further
include controlling the applied primary current and the applied
secondary current. Controlling the applied primary current and the
applied secondary current may be performed while the magnetic lens
is being used.
In an embodiment, an apparatus configured to control a magnetic
field strength of a magnetic lens is provided. The apparatus may
include a magnetic sensor disposed within a magnetic field
generated by the magnetic lens. The magnetic sensor may be
configured to generate an output signal that may be responsive to a
first magnetic field strength of the magnetic lens. The apparatus
may also include a control circuit that may be coupled to the
magnetic sensor and the magnetic lens. The control circuit may be
configured to receive the output signal from the magnetic sensor
and an input signal that may be responsive to a predetermined
magnetic field strength. A manually-controlled operating system or
a computer-controlled operating system may be coupled to the
apparatus and may be configured to generated the input signal. In
addition, the control circuit may be configured to generate a
control signal that may be responsive to the output signal and the
input signal. Furthermore, the control circuit may be configured to
apply a current to the magnetic lens. The current may be responsive
to the control signal. Therefore, the applied current may be
effective to generate a second magnetic field strength within the
magnetic lens. The second magnetic field strength may be closer to
the predetermined magnetic field strength than the first magnetic
field strength. In addition, the second magnetic field strength may
be approximately equal to the predetermined magnetic field
strength.
In an embodiment, a magnetic lens such as a magnetic lens as
described herein may be configured to apply a magnetic field to a
charged particle beam. The magnetic lens may be positioned along a
path of the charged particle beam such that the charged particle
beam may pass through the magnetic lens. The charged particle beam
may be an electron beam or an ion beam. As such, the apparatus as
described herein and the magnetic lens may be coupled to a system
such as a scanning electron microscope. The system may also include
a charged particle beam source that may be used to produce the
charged particle beam. In addition, the system may further include
a stage configured to support at least a portion of a specimen. The
stage may also be positioned along a path of the charged particle
beam such that the charged particle beam may interact with the
specimen. The apparatus and the magnetic lens may, therefore, be
used to inspect a specimen such as at least a portion of a
semiconductor device, which may be fabricated by using a
semiconductor fabrication process.
In an additional embodiment, a method for controlling a magnetic
field strength of a magnetic lens is provided. The method may be
performed substantially continuously or substantially
intermittently. The method may include generating an output signal
in response to a first magnetic field strength generated by the
magnetic lens. The method may also include receiving an input
signal. The input signal may be generated in response to a
predetermined magnetic field strength. Furthermore, the method may
include generating a control signal in response to the output
signal and the input signal. Additionally, the method may also
include applying a current to the magnetic lens. The applied
current may be applied in response to the control signal.
In a further embodiment, a method for inspecting a specimen is
provided. The specimen may include at least a portion of a
semiconductor device. The specimen may be positioned along a path
of the charged particle beam by a stage configured to support the
specimen. The method may include generating a magnetic field by a
magnetic lens and applying the magnetic field to a charged particle
beam. Applying the magnetic field to the charged particle beam may
include directing the charged particle beam through the magnetic
lens. The charged particle beam may be an electron beam or an ion
beam. The charged particle beam may be generated by using a charged
particle beam source. Therefore, the magnetic lens may be coupled
to a system such as a scanning electron microscope. In addition,
the method may include controlling a magnetic field strength of the
magnetic lens as described in the above embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention will become apparent
upon reading the following detailed description and upon reference
to the accompanying drawings in which:
FIG. 1 depicts a perspective view of an embodiment of a magnetic
lens including a pole piece having eight sectors and eight
slots;
FIG. 2 depicts a side view of an embodiment of a pole piece that
has eight sectors and eight slots;
FIG. 3 depicts a schematic view of an embodiment of a system that
includes at least one magnetic lens that has at least one pole
piece having at least two sectors;
FIG. 4 depicts a flow chart of an embodiment of a method for
applying a magnetic field to a charged particle beam using a
magnetic lens that includes at least one pole piece having at least
two sectors;
FIG. 5 depicts a schematic view of an embodiment of an apparatus
configured to control a magnetic field of a magnetic lens;
FIG. 6a depicts a flow chart of an embodiment of a method for
controlling a magnetic field of a magnetic lens in which a magnetic
sensor is disposed within a magnetic fringe field area of a
magnetic lens;
FIG. 6b depicts a schematic view of an embodiment of a magnetic
lens in which a magnetic sensor is disposed within a cavity
interposed between an outer pole piece of a magnetic lens and an
inner pole piece of the magnetic lens;
FIG. 6c depicts a schematic view of an embodiment of a magnetic
lens in which a magnetic sensor is disposed within an inner pole
piece of the magnetic lens;
FIG. 7 depicts a plot of the hysteresis of a magnetic lens using a
current feedback control apparatus; and
FIG. 8 depicts a plot of the hysteresis of a magnetic lens using a
magnetic field feedback control apparatus.
While the invention is susceptible to various modifications and
alternative forms, specific embodiments thereof are shown by way of
example in the drawings and will herein be described in detail. It
should be understood, however, that the drawings and detailed
description thereto are not intended to limit the invention to the
particular form disclosed, but on the contrary, the intention is to
cover all modifications, equivalents and alternatives falling
within the spirit and scope of the present invention as defined by
the appended claims.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawings, FIG. 1 illustrates a perspective view
of an embodiment of a magnetic lens. As used herein, a "magnetic
lens" is generally defined as a magnetic circuit configured to
apply a magnetic field to a charged particle beam. Magnetic lens 10
may have outer pole piece 12 coupled to inner pole piece 14. FIG. 2
further illustrates a side view of an embodiment of outer pole
piece 12 of magnetic lens 10. Further description of magnetic lens
10 and outer pole piece 12 will be made with respect to both FIGS.
1 and 2. A charged particle beam (not shown) may be configured to
travel through the magnetic lens along axis 16 from first end 18 of
magnetic lens 10 to second end 20 of magnetic lens 10. The charged
particle beam may include an electron beam or an ion beam. First
end 18 and second end 20 of magnetic lens 10 may form two ends of a
channel through the magnetic lens along axis 16 through which the
charged particle beam may pass during operation. The charged
particle beam may also, therefore, effectively travel through the
outer pole piece along axis 16 from a first end 22 of the outer
pole piece to second end 24 of the outer pole piece. Axis 16 of
outer pole piece 12 may be substantially the same as axis 16 of
magnetic lens 10.
Magnetic lens 10 may be configured to apply a magnetic field to the
charged particle beam in order to alter and/or control the path of
the charged particle beam through the magnetic lens. As such,
magnetic lens 10 may be configured to operate as a magnetic
objective lens that may be incorporated into any device that uses a
charged particle beam during operation. Examples of such devices
include, but are not limited to, scanning electron microscopes,
tunneling electron microscopes, e-beam lithography tools, and
focused ion beam inspection tools. Magnetic lens 10 may, however,
be incorporated into any device that uses an applied magnetic field
to alter the direction of a charged particle beam.
The two pole pieces, 12 and 14, may be coupled such that a
negligible air gap is formed between the two pole pieces.
Minimizing the air gap between the two pole pieces may be
advantageous to reduce gap deflection flux leakage, or magnetic
flux that may escape through an air gap between the pole pieces.
Such air gaps may induce eddy currents in the outer pole pieces and
the lens coil. In addition, thin circular shielding rings of a
magnetic material may be placed in a gap between the inner pole
piece and the outer pole piece of the magnetic lens to further
minimize gap deflection flux leakage.
Outer pole piece 12 and inner pole piece 14 may be formed from a
magnetically "soft" material such as ferrite, iron, holium metal,
or other suitable material having a high magnetic permeability with
minimum hysteresis. Inner pole piece 14 may be coupled to outer
pole piece 12 by screws 26, or other suitable connecting devices.
The connecting device may be made of non-magnetic materials.
Alternatively, the two pole pieces may be connected by a physically
soft material such as a silicone-based polymeric material that may
flex radially to accommodate differential thermal expansion between
the materials of the magnetic lens. Connecting the pole pieces
using a physically soft connecting material, which is non-magnetic,
may reduce fluctuations in the magnetic field of the magnetic lens
as the temperature of the magnetic lens varies due to heat
generated by the magnetic lens during operation. The variations in
temperature due to heat generated during operation may also be
reduced by cooling the magnetic lens through evaporation cooling or
by forced cooling of the lens by circulating a cooling fluid
proximate the magnetic lens.
Exterior surface 28 of outer pole piece 12 may have a substantially
conical shape tapering outward from second end 24 of the pole
piece. The conical shape of exterior surface 28 may be
advantageous, for example, to minimize the distance between the
bore, depicted as the opening in second end 20, of magnetic lens 10
and a specimen along a path of the charged particle beam.
Therefore, the distance that a charged particle beam travels
external to the applied magnetic field may be reduced, and even
minimized. Consequently, the charged particle beam that impinges
upon a specimen may more accurately reflect the characteristics of
the beam within the magnetic lens. Furthermore, the conical shape
of exterior surface 28 also increases the amount of space which is
available for other elements coupled to a system with the magnetic
lens. In a scanning electron microscope, for example, a stage
configured to support a specimen may be positioned in the path of
the electron beam proximal second end 20 of magnetic lens 10. The
stage may also be tilted to provide different angles for scanning
the charged particle beam across the sample. Therefore, a magnetic
lens having a conical shape may reduce the restrictions on the
mechanical devices that may be used in a scanning electron
microscope and the restrictions to the functioning of these
mechanical devices. Exterior surface 28, however, may also have a
substantially cylindrical shape across outer pole piece 12.
Additionally, exterior surface 28 may also have a partially conical
shape and a partially cylindrical shape. Similarly, inner pole
piece 14 may also have an exterior shape which is conical,
cylindrical, or a combination of the two shapes. The exterior
shapes of both pole pieces should be designed, however, to
facilitate connection of the pole pieces while minimizing any gap
which may be formed between the pole pieces.
Portion 30 of outer pole piece 12 may have a number of sectors 32
and a number of slots 34. Each sector 32 may be defined as the
portion 30 of outer pole piece 12 disposed between lateral
boundaries 36 of two slots 34. Slots 34 may be configured to extend
through substantially the entire thickness of outer pole piece 12,
from exterior surface 28 of outer pole piece 12 to an interior
surface (not shown) of outer pole piece 12. Slots 34, however, may
also extend partially into outer pole piece 12 from exterior
surface 28. Furthermore, first portion 38 of slots 34 may extend
through substantially the entire thickness of outer pole piece 12,
while second portion 40 of slots 34 may partially extend into outer
pole piece 12. As such, the second potion of slots 34 may provide a
space for coil windings configured to encircle each sector through
adjacent slots.
Outer pole piece 12 may have eight sectors 32 and eight slots 34.
The number of sectors on the pole piece may be larger or smaller,
however, depending upon the desired performance of the magnetic
lens. For example, a pole piece having only two sectors and two
slots may substantially enhance the performance of the magnetic
lens in comparison to the performance of a magnetic lens that does
not have sectors. Sectors 32 and slots 34 may also be formed in
inner pole piece 14 (not shown). In this manner, outer pole piece
12, inner pole piece 14 or both pole pieces of magnetic lens 10 may
have a number of sectors and slots. Sectors 32 and slots 34 may be
arranged around substantially the entire portion 30 of outer pole
piece 12. Alternatively, sectors 32 and slots 34 may be arranged
around only a fraction of portion 30 of outer pole piece 12.
Slots 34 may be spaced evenly around outer pole piece 12. For
example, if outer pole piece 12 has eight slots, then a slot may be
located every forty five degrees on outer pole piece 12.
Alternatively, slots 34 may also have an uneven spatial arrangement
around outer pole piece 12. Lateral boundaries 36 surrounding slots
34 define a lateral length 42 of each slot. In this manner, each
slot 34 may have substantially the same lateral length 42. Lateral
length 42 of each slot 34 may also be substantially smaller than a
lateral length 44 of sectors 32. Lateral length 44 of sectors 32
may be defined as a length of each sector 32 disposed between two
slots 34, and the lateral length 44 of each sector 32 may also be
substantially equal. Alternatively, lateral length 44 of each
sector 32 may vary from sector to sector.
Slots 34 may extend vertical length 46 from second end 24 of outer
pole piece 12 across portion 30 of outer pole piece 12. Vertical
length 46 of slots 34 may be larger or smaller depending on the
design characteristics of magnetic lens 10. Each slot 34 may have
substantially the same vertical length 46, and vertical length 46
may be substantially equal to vertical length 48 of each sector 42.
Vertical length 48 of sectors 32 may also be defined as a length of
sector 32 extending across portion 30 of outer pole piece 12 from
second end 24 of the pole piece. Therefore, each sector 32 may also
have a vertical length 48 that may be substantially equal to
vertical length 48 of the other sectors.
A primary coil winding (as shown in FIG. 6a as primary coil winding
134 disposed within magnetic lens 110) may also be disposed within
magnetic lens 10 and may be interposed between outer pole piece 12
and inner pole piece 14. As such, the primary coil winding may be
disposed within a cavity formed between outer pole piece 12 and
inner pole piece 14. The primary coil winding may be configured to
generate a magnetic potential, .PHI..sub.o, of outer pole piece 12
relative to a magnetic potential of inner pole piece 14 when a
current, I.sub.o, is applied to the primary coil winding. The
primary coil winding may be a number of turns of a conductive wire.
The number of turns of the conductive wire may be approximately 100
to approximately 1000. The number of turns of the coil may be
larger or smaller, however, depending on, for example, the intended
use for the magnetic lens. The primary coil winding may be formed
of an electrically conductive wire, such as copper, anodized
aluminum, or other suitable material. The primary coil winding may
also be cooled by various means during operation in order to reduce
the heat generated by the coil by evaporation cooling or by forced
cooling involving circulating a cooling fluid proximate the primary
winding coil.
Magnetic lens 10 may also have a number of sector coil windings 50.
Each sector coil winding 50 may be coupled to one sector 32 of
magnetic lens 10. Each sector coil winding 50 may be wound through
opening formed in outer pole piece 12 by two adjacent slots 34. As
such, each sector coil winding 50 may be arranged to encircle one
sector 32 of the magnetic lens. The sector coil winding may,
therefore, encircle a portion of the sector or substantially the
entire sector and may also include a number of turns of a
conductive wire. The number of turns of the conductive wire may be
configured to encircle the sector in a direction substantially
perpendicular to the path of the charged particle beam through the
magnetic lens. The number of turns for each sector coil winding may
be smaller than the number of turns for the primary coil winding.
The sector coil winding may also be formed of an electrically
conductive material and may also be cooled during operation as
described above.
The magnetic potential of each sector 32 may be first established
by the magnetic potential .PHI..sub.o, of outer pole piece 12
generated by the primary coil winding. Therefore, when a current is
applied to the primary coil winding, a magnetic potential of outer
pole piece 12 relative to inner pole piece 14 may be generated,
which may be applied approximately equally to each sector 32 of
magnetic lens 10. Current, I.sub.i, may also be passed through each
sector coil winding 50 to generate a magnetic potential,
.PHI..sub.i, for each sector 32 of magnetic lens 10. A magnetic
potential may, therefore, also be individually generated on each
sector 32 of magnetic lens 10. The resulting magnetic potential on
the i.sup.th sector, .PHI..sub.i, may, therefore, be approximately
equal to the sum of the magnetic potential resulting from the
current applied to the primary coil winding, .PHI..sub.o, and the
magnetic potential resulting from the current applied to each
sector coil winding, .DELTA..PHI..sub.1, or .PHI..sub.i
=.PHI..sub.o +.DELTA..PHI..sub.i, where .DELTA..PHI..sub.i is
approximately proportional to the current applied to the sector
winding, I.sub.i. As such, the magnetic field which may be applied
to the charged particle beam may include the magnetic potential of
outer pole piece 12 relative to inner pole piece 14 and a magnetic
potential of each sector 32 of outer pole piece 14.
In addition, current I.sub.i passed through each sector coil
winding 50 may include the zero, first, and second order harmonics
which may be expressed by the following equation: I.sub.i =A*cos
(0*i)+B*cos (.pi./4*i+.beta.)+C*cos (.pi./2*i+.gamma.). The
quantity "A" may represent the magnitude of the fine adjustment of
the focus strength of the magnetic lens which is applied to the
magnetic lens by the magnetic potential generated by the i.sup.th
sector. The quantity "B" may represent the amount of magnetic axis
displacement in the magnetic lens which is applied to the magnetic
lens by the magnetic potential generated by the i.sup.th sector,
and ".beta." may represent the direction of the magnetic axis
displacement. The quantity "C" may represent the strength of the
stigmation of the magnetic lens which is applied to the magnetic
lens by the magnetic potential generated by the i.sup.th sector,
and ".gamma." may represent the direction of the stigmation axis.
Therefore, the magnetic potential generated on each sector of the
pole piece may be used to adjust the focus strength of the magnetic
lens, the amount of magnetic axis displacement of the magnetic
lens, and/or the strength of stigmation of the magnetic lens. In
this manner, a current may be applied to each sector coil winding
of the magnetic lens to alter the magnetic potential applied to the
charged particle beam.
The performance of a magnetic lens may be substantially enhanced by
incorporating sectors on at least one pole piece of the magnetic
lens. For example, altering the magnetic potential of the magnetic
lens by generating individual magnetic potentials for each sector
that are coupled to separate sector coil windings may enable the
symmetry of magnetic lenses to be broken in a controlled fashion.
The potential advantages of such a magnetic lens include improved
capability to make fine adjustments to the focusing strength of the
magnetic field. Furthermore, a sectored magnetic lens may enable
displacing the magnetic axis of the magnetic lens that may be
useful for the purpose of aligning the lens axis with an off-axis
charged particle beam in order to minimize aberrations in the
charged particle beam. Displacing the magnetic axis may also allow
any errors in the symmetry of the magnetic lens to be corrected in
order to obtain a symmetric magnetic field. Being able to correct
the symmetry of a magnetic lens may further reduce the rejection
rate of magnetic lenses associated with magnetic lens
manufacturing. Additionally, displacing the magnetic axis may be
useful in producing a deflection of the charged particle beam to
affect the trajectory of the charged particle beam. In this manner,
a sectored magnetic lens may provide improved control over the
landing position and the landing angle of the charged particle beam
on a specimen or a detector. A stigmator field may also be produced
using a magnetic lens having sectors on at least one pole piece,
which may correct for other stigmating forces on the charged
particle beam. Furthermore, because a sectored magnetic lens may be
operated as a multi-pole device without inserting additional
devices within the magnetic lens, the sectored magnetic lens also
provides the benefits of a multi-pole device while maintaining a
co-linear path for the charged particle beam. Therefore, an
advanced magnetic lens design is provided without increasing the
complexity of the overall optical design of a system in which the
lens is incorporated.
A magnetic lens having at least one pole piece which includes at
least two sectors may also be incorporated into a system configured
to inspect a specimen such as a semiconductor device. Inspecting
semiconductor devices is an important step in manufacturing a
semiconductor device. Inspection of semiconductor devices may
usually be performed to control and improve fabrication processes.
Inspection may be performed after individual processes have been
performed or after the entire device has been fabricated. In
addition to semiconductor devices, inspection of substantially
transparent reticles may also be performed during semiconductor
manufacturing. Reticles may be used in lithography to transfer a
pattern to a resist on a semiconductor substrate. Therefore, a
defect in or on a reticle will also be transferred to the
semiconductor device. As such, careful inspection of the reticle
may usually be performed during manufacture of the reticle itself
and during subsequent use in semiconductor device
manufacturing.
As the dimensions of devices shrink, it is becoming increasingly
difficult to successfully fabricate semiconductor devices.
Therefore, it is also becoming increasingly important to monitor,
control, and improve the performance of semiconductor fabrication
processes. Analysis of fabrication processes, such as lithography
and etch, may typically be performed by generating an image of the
device and analyzing the semiconductor process by observing the
image quality and measuring critical dimensions of features of the
device. Due to reductions in feature sizes of a semiconductor
device, an inspection tool that utilizes a charged particle beam to
generate an image of a semiconductor device may generally be used
to inspect the manufactured devices. Examples of such devices
include scanning electron microscopes, tunneling electron
microscopes and focused ion beam inspection devices.
FIG. 3 illustrates an embodiment of a system configured to inspect
a specimen. System 54 may include charged particle beam source 56,
such as an ion beam or an electron beam, configured to generate
charged particle beam 58. Charged particle beam source 56 may be
any known in the art such as a cold field emission source or a
thermal field emission source. System 54 may also include several
positioning devices 60 located along the path of charged particle
beam 58 and configured to direct the charged particle beam to a
focusing device. Positioning devices 60 may include electrostatic
or electromagnetic deflectors, beam-limiting apertures, Wien
filters, and magnetic condenser lenses. Appropriate positioning
devices, however, may vary depending upon, for example, the
intended application for the system and may include any known in
the art. The positioning devices may be configured to alter the
path of the charged particle beam such that the charged particle
beam may be substantially aligned with a first end of the magnetic
lens. In addition, the positioning devices may also be configured
to apply an initial correction to the charged particle beam to
reduce effects such as chromatic aberrations, and/or dispersion in
the energy of the charged particle beam.
System 54 may include at least one magnetic lens 62. Magnetic lens
62 may be positioned such that charged particle beam 58 may enter
the magnetic lens at a first end of the magnetic lens and may exit
the magnetic lens at a second end of the magnetic lens. A
substantially co-linear void in the magnetic lens from the first
end to the second end may provide an appropriate path for the
charged particle beam. Therefore, magnetic lens 62 may be
configured to apply a magnetic field to charged particle beam 58 as
the charged particle beam travels through the magnetic lens. At
least one pole piece of magnetic lens 62 may have at least two
sectors and at least two slots. The sectors may be disposed between
lateral boundaries of the slots in the pole piece. Magnetic lens 62
may have a primary coil winding disposed within the magnetic lens.
The magnetic lens may also include a number of secondary coil
windings, and each sector coil winding may be coupled to one sector
of the pole piece. As such, the primary coil winding may be
configured to generate a magnetic potential of the outer pole piece
relative to the inner pole piece, and each secondary coil winding
may be configured to generate a magnetic potential of one sector of
the pole piece. The magnetic field applied to charged particle beam
58 by magnetic lens 62, therefore, may include the magnetic
potential of the outer pole piece relative to the inner pole piece
and the magnetic potential of at least one sector of the pole
piece. The magnetic lens may be further configured as described
herein. The magnetic lens may be also configured to operate as a
magnetic objective lens to focus the charged particle beam onto a
semiconductor device. Focusing the charged particle beam may
include reducing aberrations in the charged particle beam and
reducing the diameter of the charged particle beam to a spot size
which is appropriate for imaging a semiconductor device.
System 54 may further include stage 64 configured to support at
least a portion of specimen 66, which may include at least a
portion of a semiconductor device formed on a semiconductor
substrate, or a product wafer. Stage 64 may be positioned along the
path of charged particle beam 58. The stage may be any mechanical
device known in the art. For example, stage 64 may include a holder
configured to engage a stud. The stud may include a horizontally
flat upper surface configured to support the specimen and a post
that may be substantially vertical to the upper surface.
Semiconductor device 66 may be attached to a stud using an
adhesive. The holder may be positioned in the path of the charged
particle beam by additional mechanical devices and may include
voids configured to engage the post of the stud. As such, the stud
may be also be positioned in the path of charged particle beam when
the stud is engaged within the holder. Alternatively, stage 64 may
have a flat surface which may support specimen 66. The flat surface
of the stage may also have small holes through which a vacuum
source may be connected. In this manner, the stage may be
configured to engage the specimen using a vacuum which may be
generated by the vacuum source during operation. The stage may also
be positioned in the path of the charged particle beam manually or
by a mechanical device controlled by a controller computer.
Specimen 66 may also be positioned in the path of charged particle
beam 58 such that the charged particle beam may interact with the
specimen to generate a secondary beam of charged particles 68.
Secondary beam of charged particles 68 may include secondary
charged particles, which may emanate from recesses of the specimen,
back-scattered charged particles, which may emanate from the
surface of the specimen, and transmitted electrons, which may pass
through the specimen, such as a substantially transparent reticle.
System 54 may also include detector 70 or a plurality of detectors
and at least one device 72 configured to direct the secondary beam
of electrons to the detectors. Device 72 may include, for example,
a Wien filter. The Wien filter, or other suitable device, may be
configured to alter the path of the secondary electrons without
affecting the path of the charged particle beam which is being
directed to the semiconductor device. Detector 70 may be a Schottky
solid state barrier detector, or other suitable detectors, and may
also be coupled to operating system 74, which may be incorporated
into the system. Operating system 74 may be configured to receive a
signal from detector 70, to analyze the signal, and to generate
information about characteristics of specimen 66 such as feature
size. Operating system 74 may also be coupled to imaging device 76,
which may be a cathode ray tube. In this manner, system 54 may also
be configured to generate image profile characteristics of specimen
66. The characteristics of the device may be used to control and/or
alter a process, which was used to fabricate the specimen.
Additional features and devices that may also be incorporated into
the system are illustrated, for example, in U.S. Pat. No. 4,928,010
to Saito et al., U.S. Pat. No. 5,241,176 to Yonezawa, U.S. Pat. No.
5,502,306 to Meisburger et al., U.S. Pat. No. 5,578,821 to
Meisburger et al., U.S. Pat. No. 5,665,968 to Meisburger et al.,
U.S. Pat. No. 5,717,204 to Meisburger et al., U.S. Pat. No.
5,869,833 to Richardson et al., U.S. Pat. No. 5,872,358 to Todokora
et al., and U.S. Pat. No. 5,973,323 to Adle and are incorporated by
reference as if fully set forth herein.
FIG. 4 illustrates an embodiment of a method for applying a
magnetic field to a charged particle beam. As shown in step 78, a
charged particle beam may be generated by supplying power to a
charged particle beam source. The charged particle beam source may
be any known in the art including a cold field emission source or a
thermal field emission source. Additionally, the charged particle
beam may include a beam of electrons or ions. The charged particle
beam source may be disposed within an optical column, which may be
held under vacuum conditions during operation. As shown in step 80,
the charged particle beam may be directed to a first end of the
magnetic lens by applying electrostatic or magnetic fields to the
charged particle beam. For example, prior to traveling through the
magnetic lens, the charged particle beam may be passed through
several positioning devices located along the path of the charged
particle beam. The positioning devices and the magnetic lens may
also be incorporated into the optical column containing the charged
particle beam source. As such, the positioning devices and the
magnetic lens may be operated while the optical column is under
vacuum conditions. Positioning devices may include electrostatic or
electromagnetic deflectors, beam-limiting apertures, Wien filters
and magnetic condenser lenses. The positioning devices may alter
the path of the charged particle beam to substantially align the
charged particle beam with a first end of the magnetic lens. In
addition, the positioning device may also apply an initial
correction to the charged particle beam to reduce effects such as
chromatic aberrations, or dispersion in the energy of the charged
particle beam.
The charged particle beam may be directed to the magnetic lens such
that the charged particle beam may travel substantially through the
magnetic lens which includes at least one pole piece. The charged
particle beam may enter the magnetic lens at a first end of the
magnetic lens and may exit the magnetic lens at a second end of the
magnetic lens. The magnetic lens may be configured as described
herein.
As shown in step 82, a magnetic field may be generated within the
magnetic lens by driving a magnetic potential of the pole piece.
For example, a first current may be applied to a primary coil
winding coupled to the magnetic lens. A second current may also be
applied to at least one sector coil winding coupled to a sector of
the magnetic lens. In this manner, a magnetic potential of at least
one sector of the magnetic lens may also be generated. Therefore,
the magnetic field applied to the charged particle beam traveling
through the magnetic lens may include the magnetic potential of the
outer pole piece relative to the inner pole piece and the magnetic
potential of at least one sector. The magnetic potential of at
least one sector of the pole piece may alter the focus strength of
the magnetic lens, the magnetic axis displacement of the magnetic
lens, and/or the strength of stigmation of the magnetic lens. In
this manner, the magnetic lens may have an enhanced capability to
reduce aberrations in the charged particle beam.
In an additional embodiment, the method, as shown in FIG. 4, may
include additional steps to focus a charged particle beam on a
specimen. The specimen may include a semiconductor device, a
feature or a level of a semiconductor device, or other suitable
specimen such as a substantially transparent reticle configured to
transfer a pattern to a resist in lithography, a photoresist layer
suitable for e-beam lithography, or a biological sample. The
specimen may also be an entire semiconductor substrate that has
been processed using a semiconductor fabrication process or a
portion of the semiconductor substrate. For example, a portion of a
semiconductor substrate may be formed by cutting the substrate into
segments at appropriate positions on the substrate. As such, a
cross-sectional portion of semiconductor device, or other
semiconductor feature of interest, may be exposed to the charged
particle beam. An appropriate portion of the specimen may also be
larger or smaller depending on the capability of the device coupled
to the charged particle beam. As shown in step 84, a specimen such
as a semiconductor device may be fabricated on a semiconductor
substrate using a semiconductor manufacturing process. The
semiconductor manufacturing process may be any process known in the
art of semiconductor manufacturing, such as lithography, etch, ion
implantation, deposition, chemical mechanical polishing, and/or
plating.
At least a portion of the specimen may be positioned in the path of
the charged particle beam prior to generating the charged particle
beam, as shown in step 86. The specimen may be positioned in the
path of the charged particle beam by placing the specimen on a
stage. The stage may be located substantially within the optical
column or proximal the optical column such that the stage and the
specimen are also under vacuum conditions during operation. The
stage may be configured to support the specimen such that a
position of the specimen in the path of the charged particle beam
may be maintained throughout a process. For example, the specimen
may be attached to a stud using an adhesive. The stud may be
configured as described herein. Alternatively, the stage may have a
flat surface configured to support substantially the entire
specimen such as a semiconductor substrate or product wafer. The
flat surface of the stage may also have holes through which a
vacuum may be pulled in order to retain the position of the
specimen in the path of the charged particle beam. The stage,
however, may also be any suitable mechanical device known in the
art. The stage may also be positioned in the path of the charged
particle beam manually or by a mechanical device that may be
controlled by a controller computer.
In an additional embodiment, the method, as shown in FIG. 4, may
include additional steps to detect at least one secondary beam of
charged particles, as shown in step 88. The secondary beam of
charged particles may be produced as a result of interactions
between the charged particle beam and the specimen. The secondary
beam of charged particles may include secondary charged particles,
back-scattered charged particles, and/or transmitted charged
particles. The secondary beam of electrons may be directed back
through the magnetic lens. A device such as a Wien filter, which
may generate an electrostatic field or a combination of
electrostatic and magnetic fields, may be used to direct the
secondary beam of charged particles to a detector or a plurality of
detectors. As such, the Wien filter direct the secondary beam to a
detector at a high angle of incidence from the original path of the
secondary beam. The Wien filter, however, may apply the field to
the secondary beam without affecting the trajectory of the charged
particle beam, which may also pass through the Wien filter. The
secondary beam of charged particles may then be detected by a
Schottky solid state barrier detector, or other suitable detector.
The detector may be configured to detect the secondary beam of
charged particles and to generate a signal responsive to the
secondary beam.
In an embodiment, an image of the specimen may be generated
subsequent to detecting the secondary beam of charged particles, as
shown in step 90. An operating system coupled to the detector may
receive a signal generated by at least one detector. The signal
from the detector may be responsive to the secondary beam of
charged particles. The operating system may include a computer such
as a personal computer or a mainframe computer, which may have an
appropriate software package to perform an appropriate set of
operations on the signal from the detector. As such, the operating
system may analyze the signal from the detector and generate an
output signal representative of an image of the specimen. An
imaging system such as a cathode ray tube, which may be coupled to
the operating system, may receive the output signal from the
operating system and generate an image of the specimen. The image
may be used to analyze physical characteristics of the specimen
such as feature size and vertical profile quality. The physical
characteristics of the device may then also be used to control
and/or improve the a process which was used to fabricate the
specimen.
In a further embodiment, the method as illustrated in FIG. 4 may
also include applying a coarse focus adjustment to the charged
particle beam, as shown in step 92. An initial magnetic potential
may be generated within the magnetic lens by applying a current to
a primary coil winding of the magnetic lens. The initial magnetic
potential may also be generated within the magnetic lens by further
applying a current to sector coil winding coupled to a sector of
the magnetic lens. The initial magnetic potential may be a function
of the magnetic potential generated by the primary coil winding and
the magnetic potential generated by the sector coil winding as
described above. The initial magnetic potential may adequately
focus, or reduce aberrations in, the charged particle beam on a
specimen in order to produce an image of the specimen. The image of
the specimen may be generated as described in the above
embodiments. For example, the image of the specimen may be
generated by detecting at least one secondary beam of charged
particles which may be result from interactions between the charged
particle beam and the specimen. The image of the specimen may be
observed using a imaging device such as a cathode ray tube.
The clarity of the image of the specimen may generally be dependent
on the aberrations such as chromatic aberration and stigmation in
the charged particle beam, which interacts with the specimen.
Therefore, the initial magnetic potential may not sufficiently
reduce aberrations in, or focus, the charged particle beam in order
to obtain an adequate image of the specimen. In step 92, a coarse
focus adjustment may be applied to the charged particle beam by
altering the magnetic potential within the magnetic lens. The
coarse focus adjustment may reduce aberrations in the charged
particle beam, which are present despite the initial magnetic
field. Altering the magnetic potential within the magnetic lens may
include altering and/or controlling the current to the primary coil
winding and the current to at least one sector coil winding using a
manually-controlled device or a controller computer. The magnetic
potential within the magnetic lens may also be altered by altering
and/or controlling the current being applied to several sector coil
windings using a manually-controlled device or a controller
computer.
The manually-controlled device and the controller computer may both
be coupled to the magnetic lens. For example, an operator may
observe the image of the specimen generated by the initial magnetic
potential of the magnetic field and may manually adjust the current
being applied to the primary coil winding and the current being
applied to at least one of the sector coil windings by using a
manually-controlled device. The manually-controlled device may
include a dial, or other suitable device, which may be coupled to
the power supply of the primary coil winding or a sector coil
winding. Alternatively, a controller computer may analyze a
gray-scale image of the specimen generated by the initial magnetic
potential of the magnetic field. The controller computer may alter
the primary current applied to the primary coil winding and the
current applied to at least one sector coil winding using a set of
predefined mathematical equations.
After the coarse focus adjustment, the current being applied to the
primary coil winding may generally remain constant. For example,
the coarse focus adjustment may adequately reduce aberrations in
the charged particle beam such that only fine adjustments to the
charged particle beam may be required. Additionally, maintaining a
constant current to the primary coil winding may simplify further
operation of the magnetic lens. In step 94, a fine focus adjustment
may also be applied to the charged particle beam by further
altering the magnetic potential within the magnetic lens. Further
altering the magnetic potential within the magnetic lens may
include altering and controlling the current to at least one sector
coil winding, or multiple sector coil windings, using a
manually-controlled device or a controller computer. The
manually-controlled device and the controller computer may both be
coupled to the magnetic lens. As such, further altering of the
magnetic potential within the magnetic lens may be performed using
the manually-controlled device or the controller computer, as
described in step 92. For example, an operator may manually adjust
the current being applied to at least one of the sector coil
windings by using a manually-controlled device. As described above,
the manually-controlled device may include a dial, or other
suitable device, which may be coupled to the power supply of at
least one sector coil winding. Additional dials, or other suitable
devices, may also be coupled to additional sector coil windings
such that the current being applied to each sector coil winding may
be controlled individually. Alternatively, a controller computer
may automatically adjust the current being applied to at least one
sector coil winding by using a set of predefined mathematical
equations. The fine focus adjustment to the charged particle beam
may include an adjustment to the magnitude of a focus strength of
the magnetic lens, an adjustment to an amount of magnetic axis
displacement of the magnetic lens, and/or an adjustment to a
strength of stigmation of the magnetic lens. The fine focus
adjustment may, therefore, generate a magnetic potential within the
magnetic lens to reduce aberrations in the charged particle beam
such that an adequate image of the specimen may be generated.
Furthermore, the methods for applying a magnetic field to a charged
particle beam may be integrated into a controller for a magnetic
lens. The controller may by a computer system configured to operate
software to control the operation of the magnetic lens. The
computer system may include a memory medium on which computer
programs for operating the magnetic lens and performing
calculations related to the data collected. The term "memory
medium" is intended to include an installation medium, e.g., a
CD-ROM, or floppy disks, a computer system memory such as DRAM,
SRAM, EDO RAM, Rambus RAM, etc., or a non-volatile memory such as a
magnetic media, e.g., a hard drive, or optical storage. The memory
medium may include other types of memory as well, or combinations
thereof. In addition, the memory medium may be located in a first
computer in which the programs are executed, or may be located in a
second different computer that connects to the first computer over
a network. In the latter instance, the second computer provides the
program instructions to the first computer for execution. Also, the
computer system may take various forms, including a personal
computer system, mainframe computer system, workstation, network
appliance, Internet appliance, personal digital assistant (PDA),
television system or other device. In general, the term "computer
system" may be broadly defined to encompass any device having a
processor that executes instructions from a memory medium.
The memory medium preferably stores a software program for the
operation of the magnetic lens. The software program may be
implemented in any of various ways, including procedure-based
techniques, component-based techniques, and/or object-oriented
techniques, among others. For example, the software program may be
implemented using ActiveX controls, C++ objects, JavaBeans,
Microsoft Foundation Classes (MFC), or other technologies or
methodologies, as desired. A CPU, such as the host CPU, executing
code and data from the memory medium includes a device configured
to create and execute the software program according to the methods
described above.
Various embodiments further include receiving or storing
instructions and/or data implemented in accordance with the
foregoing description upon a carrier medium. Suitable carrier media
include memory media or storage media such as magnetic or optical
media, e.g., a disk or CD-ROM, as well as signals such as
electrical, electromagnetic, or digital signals, conveyed via a
communication medium such as networks and/or a wireless link.
The software for the magnetic lens may be used to control the
magnetic field applied to a charged particle beam. Preferably,
predefined mathematical equations that describe the relationships
between the magnetic potentials of the pole piece, the sectors of
the pole piece and the current applied to each coil winding of the
magnetic lens may be incorporated into the software. The software
may be configured to measure a magnetic field generated within a
magnetic lens. The magnetic lens may be configured as described
herein. The software may also be configured to determine a primary
current in response to the measured magnetic field, which may be
applied to a primary coil winding coupled to a pole piece of the
magnetic lens. The software may be further configured to determine
a secondary current in response to the measured magnetic field,
which may be applied to at least one secondary coil winding coupled
to a sector of the outer pole piece. Additionally, the software may
be configured to control the applied primary current and the
applied secondary current. In this manner, the software may be
configured to maintain a predetermined magnetic field within the
magnetic lens or to correct for magnetic field drift within the
magnetic field of the magnetic lens. Magnetic field drift may occur
due to the performance of the electron beam source, hysteresis
effects within the magnetic lens, temperature dependent properties
of the magnetic lens, and/or drift caused by sources external to
the magnetic lens.
FIG. 5 illustrates an embodiment of an apparatus configured to
control a magnetic field strength, or the magnetic flux density, of
a magnetic lens. Apparatus 96 may include magnetic sensor 98 that
may be disposed within a magnetic field generated by magnetic lens
100. Apparatus 96 may also include control circuit 102 which may be
coupled to magnetic sensor 98 and magnetic lens 100. The apparatus
may be configured to continuously control the magnetic field
strength of the magnetic lens. Alternatively, the apparatus may be
configured to intermittently control the magnetic field strength of
the magnetic lens. For example, the apparatus may be configured to
alter and/or control the magnetic field strength of the magnetic
lens approximately once per second. The magnetic lens may be
configured and used as described herein. For example, magnetic lens
100 may be configured to apply a magnetic field to a charged
particle beam, and the charged particle beam may be configured to
travel through the magnetic lens. Magnetic lens 100 may also be
configured to operate as a magnetic condenser lens or a magnetic
objective lens. Therefore, the magnetic lens may be configured to
reduce aberrations in a charged particle beam, such as chromatic
aberration and stigmation. The apparatus, however, may also be
coupled to any magnetic lens configured to generate a magnetic
field when a current is applied to the magnetic lens.
Apparatus 96 may be coupled to a magnetic lens and to a system,
which may utilize the magnetic lens during use, such as a scanning
electron microscope, a tunneling electron microscope, a focused ion
beam device, or any other system which may be configured to inspect
or fabricate a specimen such as a semiconductor device using a
charged particle beam. The semiconductor device may be fabricated,
prior to inspection, using a semiconductor manufacturing process,
such as lithography, etch, ion implantation, deposition,
chemical-mechanical polishing or plating. Additionally, the
semiconductor device may be a portion of a device that may be
formed on a semiconductor substrate. Alternatively, the
semiconductor device may be a working device which may be formed on
a semiconductor substrate during semiconductor manufacturing. The
magnetic lens, however, may also be coupled to any system which
utilizes a charged particle beam during operation such as an e-beam
lithography system.
The system may include at least one magnetic lens. The magnetic
lens may be configured to apply a magnetic field to a charged
particle beam. The system may also include a charged particle beam
source configured to produce the charged particle beam. The system
may be further configured such that the charged particle beam may
be configured to travel through the magnetic lens prior to
interacting with the specimen. In addition, the system may include
a stage configured to support at least a portion of the specimen.
The stage may also be positioned along the path of the charged
particle beam such that the charged particle beam may interact with
the specimen. The system may also include any of the devices as
described and shown in FIG. 3 including, but not limited to,
electrostatic or electromagnetic deflectors, beam-limiting
apertures, Wien filters, magnetic condenser lenses, Schottky solid
state barrier detectors, an operating system, and an imaging
device.
Magnetic sensor 98 may be configured to generate an output signal
responsive to a magnetic field strength of magnetic lens 100. Any
signal that may be responsive to a magnetic field strength or
another condition or property of the magnetic lens may be a
mathematical representation of the magnetic field strength or
another condition or property of the magnetic lens. For example, a
signal may be linearly, proportionally, inversely, or
logarithmically related to a condition or property of the magnetic
lens. The magnetic sensor may be configured to operate as a
Hall-effect sensor and may provide an output signal (e.g., a
voltage). The output signal may be linearly and proportionally
related to the magnetic field strength within the magnetic lens.
The output signal of magnetic sensor 98 may be an analog signal or
a digital signal. Examples of suitable Hall-effect sensors may be
commercially available from Allegro Microsystems, Inc. of
Worcester, Mass. Other suitable magnetic sensors, however, may also
be used, which provide an output signal responsive to the magnetic
field strength of the magnetic lens. The magnetic sensor may be
disposed within a magnetic fringe field area of the magnetic lens,
as shown in FIG. 5. Alternatively, the magnetic sensor may be
disposed within a cavity of the magnetic lens. The cavity may be
interposed between an inner pole piece of the magnetic lens and an
outer pole piece of the magnetic lens. Additionally, the cavity may
be disposed within the inner pole piece of the magnetic lens.
The apparatus may also include a temperature sensor to measure
temperature variations of the magnetic lens. In an embodiment,
apparatus 96 may include temperature sensor 104 coupled to the
magnetic lens and the magnetic sensor. Temperature sensor 104 may
be a matched temperature-dependent circuit element. Other suitable
temperature sensors, however, may also be included in the
apparatus. Temperature sensor 104 may be configured to monitor the
temperature of the magnetic lens and to generate an output signal,
or a temperature signal responsive to a temperature of the magnetic
lens. Therefore, magnetic sensor 98 may be further configured to
receive the temperature signal from temperature sensor 104. In this
manner, temperature sensor 104 may be positioned proximal magnetic
sensor 98 such that magnetic sensor 98 and temperature sensor 104
may be exposed to an approximately equal temperature of the
magnetic lens 100. Furthermore, magnetic sensor 98 may be further
configured to generate an output signal responsive to the magnetic
field strength of the magnetic lens in addition to the temperature
of the magnetic lens. As such, the output signal of magnetic sensor
98 may be altered to include variations in the temperature of the
magnetic lens.
Control circuit 102 coupled to the magnetic sensor may be
configured to receive the output signal from magnetic sensor 98.
The control circuit may also include a low-pass filter element (not
shown) configured to receive the output signal from the magnetic
sensor. The low-pass filter element may also be configured to
reduce fluctuations in the output signal from magnetic sensor 98.
Therefore, the low-pass filter element may prevent fluctuations in
the output signal from the magnetic sensor from being transferred
into output signals generated by the control circuit. The control
signal may be configured to generate an output signal in response
to the output signal from the magnetic sensor. The signals
generated by the control circuit may be used to control the
magnetic lens. As such, the low-pass filter element may prevent
fluctuations in the current supplied to the magnetic lens, which
may adversely affect the performance of the magnetic lens.
Control circuit 102 may also be configured to receive input signal
106, which may be responsive to a predetermined magnetic field
strength. An operating system (not shown) may be coupled to the
apparatus and may be configured to generate input signal 106. The
operating system may be manually-controlled or computer-controlled.
Input signal 106 may include a voltage, which may have a linear and
proportional relationship to the predetermined magnetic field
strength of the magnetic lens. The predetermined magnetic field
strength may be a variable magnetic field strength or a constant
magnetic field strength. Therefore, the predetermined magnetic
field strength may vary in response to a desired performance of the
magnetic lens, which may also vary over time. In addition, the
predetermined magnetic field strength may be constant in response
to a desired performance of the magnetic lens, which may be
sustained over a period of time. In this manner, the control
circuit may be configured to alter the magnetic field strength of
the magnetic lens or to maintain the magnetic field strength of the
magnetic lens.
The predetermined magnetic field strength may be determined by an
operator. For example, the manually-controlled operating system may
be configured to provide information, which may describe the
performance of the magnetic lens, to an operator. The operator may
determine the desired performance or function of the magnetic lens
by analyzing the information about the performance of the magnetic
lens. The manually-controlled operating system may be further
configured to receive an input signal from the operator
representative of a desired performance or function of the magnetic
lens. The input from the operator may also be representative of a
predetermined magnetic field strength, which may enable the desired
performance of the magnetic lens. The manually-controlled operating
system may, therefore, be configured to then convert the input from
the operator to an input signal, which may be received by the
control circuit.
Alternatively, the predetermined magnetic field strength may be
determined by a computer-controlled operating system. The
computer-controlled operating system may include a computer such as
a personal computer or a mainframe computer. For example, the
computer-controlled operating system may be configured to receive
information which may be descriptive of the performance of the
magnetic lens. The computer-controlled operating system may be
further configured to analyze the information and to generate an
input signal representative of a desired performance or function of
the magnetic lens. The input from the computer-controlled operating
system may, therefore, be representative of the differences between
the desired performance of the magnetic lens and the current
performance of the magnetic lens. As such, the input signal from
the computer-controlled operating system may also be representative
of a predetermined magnetic field strength, which may enable the
desired performance or the desired function of the magnetic
lens.
Control circuit 96 may also be configured to generate a control
signal responsive to the output signal from magnetic sensor 98 and
input signal 106. In addition, control circuit 96 may be configured
to generate a control signal responsive to the output signal and
the input signal. The control circuit may also include operational
amplifier 108 configured to receive the output signal from magnetic
sensor 98 and input signal 106. The operational amplifier may be
further configured to generate a comparison signal, which may be
responsive to differences between the output signal from the
magnetic sensor and the input signal from the operating system. For
example, the operational amplifier may perform any number of
comparisons between the output signal and the input signal
including, but not limited to, subtraction, multiplication,
division, and algorithms. Operational amplifier 108 may also be
configured to generate a control signal, which may be a function of
the comparison signal. For example, the operational amplifier may
be configured to compare the output signal from the magnetic sensor
and the input signal and to apply a gain to a difference between
the two signals. Alternatively, the control circuit may include any
circuit element or a plurality of circuit elements, which may be
configured to perform the operations described herein.
Control circuit 96 may also be configured to drive magnetic lens
100 by applying a current to at least one coil of the magnetic
lens. The current may be responsive to the control signal, which
may be generated by the control circuit. The applied current may be
effective to generate a magnetic field strength within the magnetic
lens that is closer to the predetermined magnetic field strength
than the measured magnetic field strength. Alternatively, the
applied current may be effective to generate a magnetic field
strength within the magnetic lens that is substantially equal to
the predetermined magnetic field strength. Control circuit 96 may
also include an electronic current drive system (not shown), which
may be configured to receive the control signal from the control
circuit and to drive the magnetic lens by applying a current to at
least one coil of the magnetic lens. As such, the electronic
current drive system may receive the control signal from
operational amplifier 108, which may be included in control circuit
96. In addition, the electronic current drive system may be
configured to perform an operation on the control signal. For
example, the electronic current drive system may be configured to
convert the control signal from an analog signal to a digital
signal. The electronic current drive system may be further
configured to alter and control a power source in response to the
control signal. The power source may be configured to supply a
current to at least one coil of the magnetic lens. Therefore, the
current, which may be supplied to the magnetic lens, may be altered
and/or controlled by the control circuit.
FIG. 6a illustrates an embodiment of a method for controlling a
magnetic field strength of a magnetic lens. The method may be used
to continuously or to intermittently control the magnetic field
strength of a magnetic lens. Prior to performing the method, a
magnetic field may be generated within magnetic lens 110 by
applying a current to at least one coil winding such as primary
coil winding 134 of the magnetic lens. The current may be a current
applied to magnetic lens 110 prior to beginning the method such as
a current appropriate for starting up a magnetic lens or a system
in which the magnetic lens is used. The current may also be a
current applied to magnetic lens 110 prior to beginning the method
such as a current appropriate for a prior use of the magnetic lens.
As shown in step 112, the method may include generating an output
signal responsive to the magnetic field strength of magnetic lens
110. The output signal may be generated by using magnetic sensor
114. The output signal generated by the magnetic sensor may be a
voltage, which may have a linear relationship to the magnetic field
strength of the magnetic lens.
Magnetic sensor 114 may be disposed within a magnetic field of
magnetic lens 110. For example, magnetic sensor 114 may be disposed
within a magnetic fringe field area of magnetic lens 110, as shown
in FIG. 6a. Alternatively, magnetic sensor 114 may be disposed
within a cavity of the magnetic lens. The cavity may be defined as
a space between an outer pole piece of the magnetic lens and an
inner pole piece of the magnetic lens. As shown in FIG. 6b,
magnetic sensor 114 is disposed between an outer pole piece and an
inner pole piece of magnetic lens 110. In addition, magnetic sensor
114 may be disposed within an inner pole piece of the magnetic
lens. As shown in FIG. 6c, magnetic sensor 114 is disposed within
an inner pole piece of magnetic lens 110. In this manner, magnetic
sensors may be located at different positions internal and external
to the magnetic lens.
In an embodiment, the method may include generating an output
signal which may be responsive to a temperature of the magnetic
lens, as shown in step 116. The temperature signal may be
generating by using temperature sensor 118. The generated
temperature signal may be responsive to a temperature of magnetic
lens 110. The temperature sensor may be coupled to the magnetic
lens and the magnetic sensor. Furthermore, the temperature sensor
may send the generated temperature signal to the magnetic sensor.
As a result, the magnetic sensor may alter the output signal of the
magnetic sensor in response to the temperature signal. For example,
the temperature signal may be used to alter the output signal of
the magnetic sensor such that the magnetic sensor may be sensitive
to fluctuations in the temperature of the magnetic lens. As such,
temperature sensor 118 may be positioned proximal magnetic sensor
114. In this manner, the temperature signal generated by
temperature sensor 118 may be responsive to a temperature of
magnetic lens 110 proximal magnetic sensor 114 and temperature
sensor 118. For example, as shown in FIG. 6a, temperature sensor
118 and magnetic sensor 114 may be placed within a magnetic fringe
field area of magnetic lens 110. Alternatively, as shown in FIG.
6b, temperature sensor 118 and magnetic sensor 114 may be disposed
within a cavity of the magnetic lens. In addition, as shown in FIG.
6c, temperature sensor 118 and magnetic sensor 114 may be disposed
within an inner pole piece of magnetic lens 110.
In an additional embodiment, the method may include reducing
fluctuations in the output signal, as shown in step 120. The output
signal may be sent to a low-pass circuit element, which may be
coupled to a control circuit. Reducing fluctuations in the output
signal may provide improved control on the magnetic lens by
eliminating erratic changes in the current being applied to at
least one coil of the magnetic lens. In addition, the method may
include sending the output signal to a control circuit, as shown in
step 122. The control circuit may be coupled to the magnetic sensor
and the magnetic lens.
In an embodiment, the method may include sending an input signal to
the control circuit, as shown in step 124. The input signal may be
responsive to a function of a predetermined magnetic field
strength. For example, the predetermined magnetic field strength
may be representative of a desired function or operating
characteristic of the magnetic lens. Alternatively, the
predetermined magnetic field strength may also be a substantially
constant magnetic field strength. In this manner, the method may be
used to eliminate variations in the magnetic field strength of the
magnetic lens over time. The input signal may be a voltage, which
may have a linear relationship to the predetermined magnetic field
strength of the magnetic lens. The input signal may be generated by
using an operating system, which may be coupled to the control
circuit. The operating system may be manually-controlled or
computer-controlled. As such, the input signal which may be
determined by an operator or by a controller computer.
As shown in step 126, the method for controlling the magnetic field
strength of a magnetic lens may include generating a control
signal. The control signal may be responsive to a function of the
output signal and the input signal. In an embodiment, an
operational amplifier may be coupled to the control circuit.
Therefore, generating the control signal may include generating a
comparison signal by using the operational amplifier. The
operational amplifier may be configured to generate the control
signal by comparing the output signal from the magnetic sensor and
the input signal. As such, the operational amplifier may generate a
comparison signal, which may be responsive to differences between
the output signal and the input signal. The operational amplifier
may also be configured to perform a function on the generated
comparison signal. For example, the operational amplifier may
generate the control signal by applying a gain to the comparison
signal.
In an embodiment, the method may also include sending the control
signal to an electronic current drive system, as shown in step 128.
The electronic current drive system may be coupled to the control
circuit. The electronic current drive system may be used to control
the current, which may be applied to at least one coil of the
magnetic lens. In an embodiment, as shown in step 130, the method
may include applying a current to at least one coil of the magnetic
lens. The current may be responsive to the control signal which may
be generated by the control circuit. Therefore, as shown in step
132, the predetermined magnetic field strength may be generated
within the magnetic lens. For example, the current may be effective
to generate a magnetic field strength within the magnetic lens that
is closer to the predetermined magnetic field strength than a first
or measured magnetic field strength. In addition, the current may
also be effective to generate a magnetic field strength within the
magnetic lens that is substantially equal to the predetermined
magnetic field strength.
In an embodiment, the method for controlling a magnetic field
strength of a magnetic lens may also include directing a charged
particle beam through the magnetic lens. The charged particle beam
may be an electron beam or an ion beam. In this manner, the
magnetic lens may be used to apply a magnetic field to the charged
particle beam. As such, the magnetic lens may be coupled to a
device, which may use a magnetic lens to alter the path of a
charged particle beam. Examples of such devices may include, but
are not limited to, scanning electron microscopes, tunneling
electron microscopes, e-beam lithography devices or focused ion
beam devices. For example, the magnetic lens may be configured to
operate as a magnetic lens incorporated into a scanning electron
microscope.
Inspecting a specimen such as a semiconductor device, which may be
formed on a semiconductor substrate, subsequent to intermediate and
final processing steps has become an integral part of successfully
producing working semiconductor devices. The semiconductor device
may be fabricated prior to inspection by using a semiconductor
manufacturing process. The semiconductor manufacturing process may
include a number of processing steps as described herein. The
semiconductor device may also include a semiconductor topography,
which may be only a portion of a working semiconductor device. For
example, a semiconductor device may be inspected subsequent to each
of the many processing steps required to fabricate a working
semiconductor device.
In an embodiment, the method for inspecting a specimen may include
generating a magnetic field within the magnetic lens by applying a
current to at least one coil of the magnetic lens. In addition, the
method may also include applying the generated magnetic field to a
charged particle beam by directing the charged particle beam
through the magnetic lens. The charged particle beam may be an
electron beam or an ion beam. The method may also include
generating the charged particle beam by using a charged particle
beam source. Prior to directing the charged particle beam through
the magnetic lens, at least a portion of a specimen may be
positioned on a stage configured to support the specimen. The stage
may be located along a path of the charged particle beam. As such,
the charged particle beam may impinge upon the specimen after
passing through the magnetic lens. The charged particle beam
exiting the magnetic lens may be substantially free of aberrations.
Therefore, the magnetic lens may be coupled to a scanning electron
microscope or other suitable device, which may apply a magnetic
field to a charged particle beam to alter a path of the charged
particle beam, or to reduce aberrations that may be present in the
charged particle beam.
Inspecting a semiconductor device may also include monitoring and
altering a magnetic field strength of the magnetic lens. Monitoring
and altering the magnetic field strength of the magnetic lens may
include generating an output signal, which may be responsive to a
magnetic field strength of the magnetic lens by using a magnetic
sensor. The magnetic sensor may be disposed within a magnetic field
of the magnetic lens. The method may also include sending the
output signal to a control circuit, which may be coupled to the
magnetic sensor and the magnetic lens. In addition, the method may
include sending an input signal to the control circuit. The input
signal may be responsive to a function of a predetermined magnetic
field strength. The method may also include using the control
circuit to generate a control signal which may be responsive to a
function of the output signal and the input signal. Furthermore,
the method may include generating the predetermined magnetic field
strength within the magnetic lens by applying a current to at least
one coil of the magnetic lens. The current may be responsive to the
control signal. The method may also include additional steps as
described herein.
The apparatus and methods for using the apparatus described above
may provide accurate control of the magnetic field within a
current-driven magnetic lens. The magnetic lens may be coupled to
any device which uses a charged particle beam to perform a
function. Using the apparatus and methods to control a magnetic
field of a magnetic lens may eliminate adverse effects of
hysteresis on the performance of the magnetic lens. In addition,
the apparatus and methods may also reduce the effects of
temperature dependent material properties, drift in current drive
electronics, low frequency noise, eddy currents, undesired
superimposed fields on the magnetic field generated by a magnetic
lens. Furthermore, the apparatus and methods described above may
also be used to eliminate drift in the magnetic field strength over
time from other causes. In the application to charged particle beam
devices, this functionality may be useful for reproducibly tuning
magnetic components as magnetic lenses, Wien filters, or deflection
coils. Specifically, the magnetic field sensor may permit easier
manual and automated operation of scanning electron microscopes and
other electron beam devices. The magnetic feedback concept may
greatly increase tool stability and tool to tool consistency.
Specifically, optimized values of magnetic field for deflection
coils, Wien filters and magnetic lenses may be readily reproduced.
Additionally, using a magnetic field sensor may eliminate the need
to couple a current sensing resistor to the control circuit.
Therefore, the magnetic lens may be driven with the same current
but by applying a lower voltage to the magnetic lens.
EXAMPLE
Effect of Magnetic Field Feedback Control on a Magnetic Lens
FIG. 7 illustrates a plot of the hysteresis of a magnetic lens
using a current feedback control apparatus. The magnetic field
strength was estimated by measuring the current in the lens coil.
The current in the lens coil was varied to alter the magnetic field
strength of the magnetic lens. Variations in the current in the
lens coil were highest at the beginning of testing and were
decreased over time. The first current, which was applied to the
lens coil, has been designated as the point on the graph which has
been labeled "start." The last current, which was applied to the
lens coil, has been designated as the point on the graph which has
been labeled "end." The measured magnetic field has been plotted
versus the desired magnetic field. The range of the measured
magnetic field and the desired magnetic field range from
approximately -10 gauss to approximately +10 gauss. As shown in
FIG. 7, the measured magnetic field has a non-linear relationship
to the desired magnetic field. The non-linear relationship may be
caused by the fact that instantaneous magnetic field strength is
determined by the present current and the history of all other
currents in the coil and is commonly referred to as hysteresis.
FIG. 8 illustrates a plot of the hysteresis of a magnetic lens
using a magnetic field feedback control apparatus. The magnetic
field strength was estimated by measuring the magnetic field of the
magnetic lens using an Allegro Hall Sensor (Allegro Microsystems,
Inc., Worcester, Mass.). The current in the lens coil was varied to
alter the magnetic field strength of the magnetic lens. Variations
in the current in the lens coil were highest at the beginning of
testing and were decreased over time. The measured magnetic field
has been plotted versus the desired magnetic field. The range of
the measured magnetic field and the desired magnetic field is from
approximately -10 gauss to approximately +10 gauss. As shown in
FIG. 8, by using a magnetic field feedback control apparatus, a
linear relationship between the measured magnetic field and the
desired magnetic field was established. The linear relationship
between the measured magnetic field and the desired magnetic field
indicates that the effects of hysteresis in the magnetic lens were
effectively minimized. Therefore, by implementing a control method,
which includes using a magnetic field feedback control apparatus as
described herein, hysteresis in the magnetic field strength of the
magnetic lens may be substantially eliminated.
It will be appreciated to those skilled in the art having the
benefit of this disclosure that this invention is believed to
provide a magnetic lens having at least one pole piece which has at
least two sectors and an apparatus configured to control the
magnetic field strength of a magnetic lens. Further modifications
and alternative embodiments of various aspects of the invention
will be apparent to those skilled in the art in view of this
description. For example, the structure of a magnetic lens may also
be applied to electrostatic devices, such as deflectors, or
devices, which generate both magnetic and electrostatic potentials,
such as Wien filters. In addition, the apparatus, which may monitor
and control a magnetic lens using magnetic field feedback control,
may be integrated into any device which generates a magnetic field
during operation. It is intended that the following claims be
interpreted to embrace all such modifications and changes and,
accordingly, the specification and drawings are to be regarded in
an illustrative rather than a restrictive sense.
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